As standard the 1800 doesn't have a belly pan, it's just got some plastic trim running around the sides (around the exhaust headers). If you have a belly pan, it has to be an after market item and they usually mount with 4 screws (usually screws that pass the rough the plastic side trims and into the belly pan. If you post a picture we can tell you who made the belly pan and give you more exact instructions on how to remove it.

Your belly pan is a after market add on. it is attached using the four lower acorn nuts that hold the bottom side panels. Take off the nuts. Pull the side panels off the studs and then the belly pan off the stud. do the same for the other side. Just reverse the proceedure to put it all back together.